1. Field of the Invention
The present invention generally relates to a transmitter, and more particularly, to a transmission architecture of a transmitter.
2. Description of Related Art
A transmitter in the digital communication and broadcasting system transmits an information signal to a receiver through a channel in the form of electromagnetic waves. However, due to undesirable channel effects like multi-path reflection and signal attenuation, the received signal may be distorted. If there is a long time interval between the received multi-path signals, the delay spread may be enlarged. Meanwhile, the reciprocal value of the delay spread is approximate to a coherent bandwidth, and the channel frequency response thereof may cause a frequency-selective fading effect. In the orthogonal frequency division multiplexing (OFDM) transmission technology of multi-carrier modulation, a guard interval (GI) is usually added at the front end of an effective symbol for counteracting the multi-path channel effect, so as to alleviate or avoid the intersymbol interference (ISI) impacts for the transmitted signals. Therefore, since the OFDM transmission technology based upon multi-carrier modulation is capable of effectively processing the multi-path channel effect, it has gradually become a mainstream technology in the applications of wired/wireless communication and digital broadcasting in recent years.
The network construction of the OFDM system can be classified into multi-frequency network (MFN) and single frequency network (SFN). The SFN has the following three apparent advantages. First, the SFN merely requires a small power but provides many distribution points, and thus covering a wide range. Second, the SFN saves frequency resources, that's because the whole system only requires one frequency. Third, when a user moves within the coverage area, the receiver in operation need not change the frequency. Therefore, most of the systems employ the SFN architecture to completely use the allocated frequency bands. The OFDM systems adopting the SFN architecture include digital video broadcasting-terrestrial (DVB-T), digital video broadcasting-handheld (DVB-H), digital audio broadcasting (DAB), terrestrial digital multimedia television broadcasting (DMB-T), and multimedia forward link only (FLO).
Besides the characteristic of counteracting the multi-path channel effect, the OFDM system further has the functions of channel encoding and signal interleaving, such that the consecutive errors caused by the channel effect during the transmission process can be eliminated, and when receiving signals, the error bits are corrected with correct ones based on the channel decoding and signal de-interleaving technology. However, in order to more effectively correct the error bits, besides the functions of channel encoding and signal interleaving, the channel frequency response must be diversified, such that when the signal received by the receiver becomes an error signal after passing through a part of the channels with relatively poor frequency response, the error signal can be corrected according to a correct signal generated through a part of the channels with desirable frequency response.
To ensure the diversity of the channel frequency response, the transmitter in the OFDM system is usually accomplished by a diversity transmission technique or the receiver is accomplished by a diversity receiving technique. The diversity gain generated by the diversity technology may enhance the receiving performance of the receiver.
During the usage and system building of the SFN, though the coverage area is quite large, on the boundary between the signal coverage areas of two transmitters in the SFN, the receiver may simultaneously receive identical transmitted signals from the two areas. Under such a channel environment with extremely small delay spread, the coherent bandwidth is a wideband coherent bandwidth having rather slow changes in the channel frequency response, so as to generate a flat fading channel response. At this time, if the channel frequency responses on the boundary of the signal coverage areas of the two adjacent transmitters in the SFN make the signals have inverted phases, destructive interference may occur to the whole channel frequency responses, and channel frequency responses may have a rather low energy. Further, if the fading coherent time lasts too long due to the shadow effect, the OFDM transmission technology may not show its advantage in counteracting the multi-path channel effect. Besides, even if the OFDM transmission technology has the functions of channel encoding and signal interleaving, it still cannot correct the error signal generated by the poor channel frequency response with the correct ones generated by the desirable channel frequency response under the circumstances that the channel frequency response has a too low energy and the fading coherent time is extremely long. Therefore, it has become a key point in the design of transmitter about how to process signals in the transmitter with diversity technology without affecting the original system performance of the SFN or altering the original design of the receiver.
The paper, entitled “R1-061264: Further Studio on Reference Signal Structure for MBMS” (3GPP LTE RAN1 meeting document, May 12, 2006), issued by Toshiba Corp. and NTT DoCoMo discloses that, in the transmitter for each cell of the SFN, the OFDM system can utilize different scramble parameters to encode sub-carriers in various different groups, such that the receiver may generate a diversified channel frequency response upon receiving a synthesized signal of the two transmitters on the boundary of the areas. Further, with the additional functions of channel encoding and signal interleaving, a multi-cell diversity gain is obtained. Therefore, the above technique can eliminate the disadvantage that the channel frequency response on the cell boundary has too low energy. In addition, the technique need not particularly modify the original design of the receiver, which does not increase the cost on improving the system performance.
In addition, as the OFDM system adopts the multi-carrier modulation technique, the transmitted signals may have excessive large peak-to-average power ratios (PAPRs). As a result, nonlinear distortion of signals may occur when being transmitted through a power amplifier, that is, some transmitted signals with relatively large power may be clipped. Therefore, it is also a key point in the design of the transmitter as to how to reduce the PAPRs of the signals transmitted by the transmitter.
The paper, entitled “OFDM with Reduced Peak-to-average Power Ratio by Optimum Combination of Partial Transmit Sequences”, (Electronics Letters, vol. 33, no. 5, pp. 368-369, February 1997) issued by S. H. Muller and J. B. Huber discloses a PAPR reduction method. First, each OFDM symbol (X) of a transmitted signal with a length of N samples is divided into M symbols (X1, X2, . . . , XM). In each of the symbols, only an individual part of the sub-carriers are assigned with values, and the others are zero. Each of such symbols goes through inverse discrete Fourier transformation with a length of N points and then they are respectively multiplied by a set of coefficients (b1, b2, . . . , bM). Afterwards, these products are summed up to calculate a PAPR of the summed signal. As for the same OFDM symbol (X), different sets of coefficients are used for producing different corresponding summed signals, so as to calculate a plurality of corresponding PAPRs, and then the set of coefficients corresponding to the minimum PAPR is determined. Finally, the set of coefficients and the corresponding summed signal are output. Additionally, as the set of coefficients are designed as random coefficients, they may cause difficulties in the channel estimation of the receiver if being combined with the channel effect. In order to prevent the channel estimation from being affected by the set of coefficients, the known reference signal (for example, a pilot signal) for estimating the channel frequency response is generally not processed by the set of coefficients. Therefore, the receiver needs a side information and a safer channel to transmit the side information. The side information is used for informing the receiver about the set of coefficients, so as to facilitate the receiver to restore the original OFDM symbol (X).
In view of the above, the conventional transmitter has been improved by enhancing the multi-cell diversity gain, or by reducing the PAPR. However, if the two problems occur in the transmitter at the same time, a transmitter capable of both enhancing the multi-cell diversity gain and reducing the PAPR is required.